Semipermeable membranes play an important part in industrial processing technology and other commercial and consumer applications. Examples of their applications include, among others, biosensors, transport membranes, drug delivery systems, water purification systems, optical absorbers, and selective separation systems for aqueous and organic liquids carrying dissolved or suspended components.
Generally, semipermeable membranes operate in separation devices by allowing only certain components of a solution or dispersion to preferentially pass through the membrane. The fluid that is passed through the membrane is termed the permeate and comprises a solvent alone or in combination with one or more of the other agents in solution. The components that do not pass through the membrane are usually termed the retentate. The permeate and/or retentate may provide desired product.
Reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) are examples of membrane processes. Microfiltration is a separation process that utilizes membranes having pores of sizes from about 0.1 microns to about 10 microns. Ultrafiltration is a separation process that utilizes membranes having defined pores of sizes of about 1 nm to about 0.1 microns. Ultrafiltration membranes are often characterized by their “molecular weight cutoff”, a technique that defines the ability of ultrafiltration membranes to separate polymers from a solvent. A molecular weight cut-off method is described in ASTM method E1343-90 (1997)e1: “Standard Test Method for Molecular Weight Cutoff Evaluation of Flat Sheet Ultrafiltration Membranes”.
Nanofiltration is a process where a favorable portion of at least one small agent (typically less than 1000 MW or a salt) passes through the membrane with the solvent and a desirable amount of at least one other small agent (typically less than 1000 MW or a salt) is retained. An example of a nanofiltration process is the desalting of a sugar solution, where 80% of the salt passes the membrane with the water and 95% of the sugar is retained by the membrane. In this example, the sugar and salt can be fractionated. Because nanofiltration is a process, the definition of a nanofiltration membrane is a membrane commonly used in nanofiltration processes.
Reverse osmosis is a process where the large majority of each agent in solution is retained by the membrane while the solvent passes through the membrane, with the common provision that at least one of the agents being removed in solution is small (less than 1000 MW or a salt). Examples of reverse osmosis processes are the purification of seawater, where often less than 1% of the species in the seawater are found in the permeate. Because reverse osmosis is a process, the definition of a reverse osmosis membrane is a membrane commonly used in reverse osmosis processes.
It should be well understood that a membrane commonly termed a nanofiltration membrane can be capable of reverse osmosis and vice versa. For example, a common so-called nanofiltration membrane, Desal 5 DK, can retain greater than 99% of magnesium sulfate from water. In this case, because the large majority of the magnesium sulfate is retained and the permeate contains a low amount of this salt, the process is reverse osmosis. Therefore, this is an example of a reverse osmosis process using a “nanofiltration” membrane. Also, a common reverse osmosis membrane, Desal 3 SG, can pass hydrofluoric acid with water while retaining simple ions such as sodium, copper, and chloride. In this example, the membrane discriminates between the HF and the other small agents in solution, making it a nanofiltration process using a “reverse osmosis” membrane.
The performance of RO and NF membranes typically is characterized by two parameters: permeate flux and solute rejection. The flux parameter indicates the rate of permeate flow per unit area per unit pressure of membrane. The rejection indicates the ability of the membrane to retain certain components while passing others.
RO and NF membrane processes require a pressure or concentration gradient in order to perform the desired separation. When functioning to separate, the process using a reverse osmosis membrane overcomes the osmotic pressure resulting from the differential concentration of salts across the membrane. Pressure must be applied to the feed solution being separated in order to overcome this osmotic pressure and to cause a reasonable flux of permeate. RO and NF membranes typically exhibit high flow rates or fluxes at reasonable pressures. Currently, such membrane fluxes on the order of about 1*10−5 to 50*10−5 cm3/cm2*sec*atm.
The majority of RO and NF membranes are constructed as composite membranes having a thin barrier membrane formed as a coating or layer on top of a porous support material. Typically, this RO or NF membrane is formed by interfacial polymerization of a thin film on a porous support. For example, U.S. Pat. No. 3,744,642 to Scala discloses an interfacial membrane process for preparation of a reverse osmosis membrane. Additional U.S. patents disclosing polyamide and polysulfonamide membranes include U.S. Pat. Nos. 4,277,344; 4,761,234; 4,765,897; 4,950,404; 4,983,291; 5,658,460; 5,627,217; 5,693,227; 6,783,711; and 6,837996.
Current interfacially prepared membranes substantially reach the goals of extreme thinness and substantial freedom from flaws or imperfections. The closer an RO or NF membrane comes to these two goals, the better is its flux and rejection values. These two features of minimal thickness and freedom from flaws, however, are not altogether compatible objectives. As the thickness of the polymeric film or membrane decreases, the probability increases significantly that holes or void spaces in the film structure will be formed. Of course, these holes or void spaces result in significant loss of solute rejection.
When processing conditions to form such thin and defect free membranes are found, it is often the case that changes to those conditions are detrimental to performance. As a result, much work on improved interfacial membranes has focused on ways to alter the membrane without changing the process used to initially form the membrane. One common means of affecting the character of a membrane is through the use of post treatments. Post treatments leading to improved permeability, improved rejection, and improved resistance to fouling have been disclosed previously.
Post treatments meant to improve rejection have involved reactions with amine reactive molecules. U.S. Pat. No. 4,960,517 teaches the use of amine reactive species which reduce the passage of sulfuric acid and U.S. Pat. No. 5,582,725 teaches the use of a post treatment with acyl halides after the membrane has been swollen and then redried.
There is currently a needed for new post treatment methods that can be chosen independent of the film forming reactants and can be used to selectively alter the thin film. This would enable freedom to tailor the post treatment chemistry to improve rejection or fouling characteristics of the membrane while retaining the same reactants and process conditions used to initially form the membrane. These post treatment methods should utilize reagents that are quite reactive with residual amine groups to allow rapid modification, but not highly reactive with the solvent used in the modification, for example alcohols. Such post treatments would allow a single manufacturing process to produce multiple products by alteration of the functionality present on the post treatment molecule.
The Bayer process is used industrially to recover aluminum hydroxide from bauxite. U.S. Pat. No. 4,786,482 reports the use of porous polysulfone hollow fibers coated with a semipermeable sulfonated polysulfone membrane to reduce the levels of organic and inorganic impurities in caustic liquors. Although this patent issued more than 15 years ago, membranes are not routinely used in industry for purifying highly caustic streams, because membranes having a commercially viable combination of flow, rejection, and caustic stability have not been identified. Accordingly, there is also currently a need for materials and methods that can be used to remove impurities from caustic streams, such as the caustic streams generated by a Bayer alumina recovery process.